Ethanol for industrial use is conventionally produced from petrochemical feed stocks, such as oil, natural gas, or coal, from feed stock intermediates, such as syngas, or from starchy materials or cellulosic materials, such as corn or sugar cane. Conventional methods for producing ethanol from petrochemical feed stocks, as well as from cellulosic materials, include the acid-catalyzed hydration of ethylene, methanol homologation, direct alcohol synthesis, and Fischer-Tropsch synthesis. Instability in petrochemical feed stock prices contributes to fluctuations in the cost of conventionally produced ethanol, making the need for alternative sources of ethanol production all the greater when feed stock prices rise. Starchy materials, as well as cellulosic material, are converted to ethanol by fermentation. However, fermentation is typically used for consumer production of ethanol, which is suitable for fuels or human consumption. In addition, fermentation of starchy or cellulosic materials competes with food sources and places restraints on the amount of ethanol that can be produced.
As an alternative to fermentation, ethanol may be produced by hydrogenating acetic acid and esters thereof. Ethanol production via the reduction of acetic acid generally uses a hydrogenation catalyst. The reduction of various carboxylic acids over metal oxides has been proposed.
EP0175558 describes the vapor phase formation of carboxylic acid alcohols and/or esters such as ethanol and ethyl acetate from the corresponding mono and di-functional carboxylic acid, such as acetic acid, in the presence of a copper oxide-metal oxide supported catalyst, such as CuO/ZnAl2O4. A disadvantage with copper oxide catalysts in carboxylic acid hydrogenation reactions is the lack of long-term catalyst stability.
U.S. Pat. No. 4,398,039 describes a process for the vapor phase hydrogenation of carboxylic acids to yield their corresponding alcohols in the presence of steam and a catalyst comprising the mixed oxides of ruthenium, at least one of cobalt and nickel, and optionally one of cadmium, zinc, copper, iron, rhodium, palladium, osmium, iridium and platinum. The total loading of active metals is 5%. A process is further provided for the preparation of carboxylic acid esters from carboxylic acids in the absence of steam utilizing the above-identified catalysts.
U.S. Pat. No. 4,517,391 describes preparing ethanol by hydrogenating acetic acid under superatmospheric pressure and at elevated temperatures by a process wherein a predominantly cobalt-containing catalyst is used and acetic acid and hydrogen are passed through the reactor, at from 210 to 330° C., and under 10 to 350 bar, under conditions such that a liquid phase is not formed during the process. The cobalt-containing catalyst contains, as active components, from 50 to 80% by weight of Co, from 10 to 30% by weight of Cu, from 0 to 10% by weight of Mn, from 0 to 5% by weight of Mo and from 0 to 5% by weight of phosphoric acid, the percentages being based on the metal content. However, the productivity as reported in the examples of U.S. Pat. No. 4,517,391 is 89.8 grams of ethanol per kilogram of catalyst per hour, which is too low for commercial production.
U.S. Pat. No. 4,918,248 describes producing an alcohol by catalytically reducing an organic carboxylic acid ester with hydrogen in the presence of a catalyst obtained by reducing a catalyst precursor comprising (A) copper oxide and (B) titanium oxide and/or titanium hydroxide at a weight ratio of (A) to (B) in the range between 15/85 and 65/35. The component (A) may alternatively be a composite metal oxide comprising copper oxide and up to 20 wt. % of zinc oxide.
CN103785418 discloses a catalyst containing at least cobalt and tin and its application in the preparation of alcohol by carboxylic acid hydrogenation. The disclosed catalyst contains 10 wt. % to 50 wt. % cobalt and 0.1 wt. % to 50 wt. % tin in the total weight of catalyst, in addition to other metals such as silver, zinc, and zirconium. CN103785418 prepares the hydrogenation through the co-precipitation, deposition-precipitation, steamed ammonia precipitation, sol-gel, and being dissolved as alloy, then combined with one type or multiple types of suction filtration and ball milling methods.
CN103785415 discloses a method to prepare alcohol by selective hydrogenation of carboxylic acid. The disclosed catalyst contains 10 wt. % to 50 wt. % cobalt and 0.1 wt. % to 50 wt. % bismuth in the total weight of catalyst.
CN103787827 discloses a method to prepare alcohol by selective hydrogenation of carboxylic acid. The disclosed hydrogenation catalyst contains at least cobalt and a trace amount of precious metal additives of platinum, palladium or rhenium.
Other hydrogenation catalysts that are not metal oxides have also been proposed. U.S. Pat. No. 6,495,730 describes a process for hydrogenating carboxylic acid using a catalyst comprising activated carbon to support active metal species comprising ruthenium and tin. U.S. Pat. No. 6,204,417 describes another process for preparing aliphatic alcohols by hydrogenating aliphatic carboxylic acids or anhydrides or esters thereof or lactones in the presence of a catalyst comprising platinum and rhenium. U.S. Pat. No. 5,149,680 describes catalytic hydrogenation of carboxylic acids and their anhydrides to alcohols and/or esters in the presence of a catalyst containing a Group VIII metal, such as palladium, a metal capable of alloying with the Group VIII metal, and at least one of the metals rhenium, tungsten or molybdenum. U.S. Pat. No. 4,777,303 describes the production of alcohols by the hydrogenation of carboxylic acids in the presence of a catalyst that comprises a first component which is either molybdenum or tungsten and a second component which is a noble metal of Group VIII on a high surface area graphitized carbon. U.S. Pat. No. 4,804,791 describes another production process of alcohols by the hydrogenation of carboxylic acids in the presence of a catalyst comprising a noble metal of Group VIII and rhenium.
Thus, further improvements to hydrogenation catalysts that demonstrate high stability, conversion of acetic acid, and other oxygenates, with high selectivity to ethanol are needed.